Extended Forecast
Valid: Wed 30 Dec 2009 06:00 to Thu 31 Dec 2009 06:00 UTC
Issued: Mon 28 Dec 2009 23:23
Forecaster: TUSCHY
A level 1 was issued for France and western Switzerland mainly for severe wind gusts and to a lesser extent for tornadoes.
A level 1 was issued for parts of Spain and Portugal mainly for severe wind gusts, tornadoes and to a lesser extent for excessive rainfall and large hail.
A level 1 was issued for Croatia, parts of Slovenia mainly for severe wind gusts and to a lesser extent for tornadoes.
SYNOPSIS
Positive tilted trough over the NE Atlantic starts to lift out towards the east/northeast with timing being a major issue. Latest ENS forecasts (GSF) also place the main uncertainty along the SW/W egde of the trough and delay the eastward movement from run to run. However, this won't cause major uncertainties for Europe, as SW-erly flow will cover the forecast area with numerous, embedded disturbances. The air mass features unseasonable moist and warm values and a broad area may see showers/thunderstorms over west/central Europe.
The central Mediterranean remains capped with hot desert air, whereas NE-Europe will still be covered by cold and comparatively drier air.
DISCUSSION
The active weather pattern persists through that period. Due to the extensive nature of this vortex, numerous regions over Europe may see severe weather episodes, restricted in time and space.
... France ...
A modified maritime, subtropical air mass covers the warm sector, which is nearly all of France. However, time of the year is not favorable for the BL to keep that moisture and gradual mixing is forecast with a constant decrease of the dewpoints in the PBL. In addition, trajectories show an unfavorable path over Portugal and Spain with additional drying. Hence, the thunderstorm risk may see a decrease during the peak time heating with the most prolonged period over far eastern France and west Switzerland until sunset.
A broad-brushed level 1 was issued for most parts of France, as widespread 100-locally up to 500 J/kg MLCAPE are forecast with variations in different model data. However, EZMWF,WRF and GFS agree in the release of CAPE with cold ELs and adequate forcing by short waves to expect scattered thunderstorms. Severe wind gusts are possible due to steepening LL lapse rates and 20-25m/s at 850hPa. Low LCLs and moderate directional shear at all levels also point to a tornado risk, probably maximized in the northern/eastern part of the level 1, but this risk has to be revised with new data. Current inidcations are that the level 1 may expire after sunset.
... Portugal and parts of Spain...
A prolonged period of onshore flow of a moist, maritime air mass affects the highlighted area during the forecast. Placed beneath the eastward moving upper trough axis, moderate offshore CAPE is a reasonable forecast. CAPE gradually decreases onshore with model discrepancies, how far inland the fetch will spread. The pattern favors the maximized risk along the coast, decreasing inland but also a shift of the severe risk from NW-Spain and Portugal to S-Portugal and SW-Spain betimes. The exact path of surface depressions dictates a regionally enhanced risk, which can't be considered that far out. Tornadoes, severe wind gusts, isolated large hail and excessive rainfall are all a potential risk with thunderstorm activity.
... Croatia, parts of Slovenia and Hungary ...
The time-frame of interest will be after sunset, which is not the best timing in the winter time. However, unseasonably warm and moist air spreads eastwards beneath cooling mid-levels. The model trend again (similar to the 25th event) indicates more robust CAPE from run to run, but still instability is yet too marginal for a more widespread severe risk. SBCAPE is bound to the theta-e tongue and this area was highlighted by a level 1. Futher to the east/northeast, an extensive are with MUCAPE builds eastwards, as mid-level lapse rates steepen. Speed and directional shear at all levels would be sufficient for organized thunderstorms with all facets of severe, but later model data will be evaluated, before expanding the level area eastwards. Another point of concern is the warm EL forecast in the level 1 area with decreasing values eastwards, so electrified convection may be quite limited. However, LL CAPE and very strong LL shear overlap, so any stronger updraft may pose a severe risk (e.g. downward momentum of 30m/s at 850hPa).
... Central Mediterranean ...
Thunderstorm probabilities over the north-central Mediterranean (north of a line Majorca-Corsica) increase after midnight and at least isolated thunderstorms are possible as the region gets placed beneath the cyclonic flank of a strong jet (even coupled jet configuration at mid-levels). The main risk will be severe wind gusts, but late development and limited coverage still preclude a level 1 for now.
